Migrating Slavs arrived in southeastern Europe in the 6th and 7th centuries. The Byzantine Emperor Heraclius specifically mentions a tribe called he Croats, who he employed to help defend the empire against marauding Avars.

Other slav tribes migrated into southeastern Europe at around the same time, coalescing into communities subsequently known as Slovenes, Serbs, Macedonians and Bulgarians. The newcomers settled down and lived under the suzerainty of Byzantium or other powers, although they gradually developed states of their own.

Cities on the Adriatic seaboard preserved elements of late-Roman and Byzantine culture - producing architectural masterpieces such as the Euphrasian basilica in Poreč and Church of St Donat at Zadar.